Allograft rejection is a CD4+T cell dependent process. These T cells can recognize alloantigen via two distinct, yet not mutually exclusive, pathways. In the "direct" pathway, T cells recognize intact allo-MHC molecules on the surface of donor antigen-presenting cells (APCs). In the "indirect" pathway, T cells recognize processed alloantigen (predominantly allo- MHC) presented as peptides by self APCs. Increasing evidence from experimental animals and humans supports a significant role for indirect allorecognition in mediating allograft rejection. It has been hypothesized that this pathway, analogous to the physiologic nominal antigen recognition pathway, may be important in development and progression of chronic rejection, the most important problem in clinical organ transplantation. Therefore, it can be argued that the absence of tolerance to indirect allorecognition is responsible for development of chronic rejection. Definitive experimental evidence supporting these hypotheses is lacking. The purpose of this proposal is to study the role and effector mechanisms of indirect allorecognition in mediating allograft rejection, particularly chronic rejection. The investigators will also study the effects and mechanisms of inhibiting indirect allorecognition on development of the rejection process. In the first specific aim they will determine whether priming animals with donor- derived MHC allopeptides induces/accelerates allograft rejection. They will also study whether inhibiting CD4+T cell activation via the indirect pathway prevents development of acute and chronic rejection. In specific aim 2 the investigators will use established Th1 and Th2 T cell clones which are self-restricted to recognize and respond to donor class II MHC allopeptides to study whether adoptive transfer of such clones will "promote/enhance" (Th1) or "regulate" (Th2) the immune response to vascularized allografts. Finally, in specific aim 3, in collaboration with the laboratory of Dr. Laurence A. Turka, they plan to create a TCR transgenic animal with specificity to donor class II MHC allopeptide presented by self APCs. This animal, when backcrossed onto SCID or RAG2 knockout mice, can only reject an allograft by indirect allorecognition. The above studies are critical to understanding of the contribution and mechanisms of indirect allorecognition in mediating acute and chronic allograft rejection. Results from these studies should yield clinically relevant information facilitating development of novel strategies to induce donor-specific tolerance.